Moonmail's promises of a serverless architecture and a GraphQL API are interesting, but if you take a look under the covers you can tell that it's not a well-maintained project:
This paints a clear and unfortunate picture: the software is abandoned. If you're looking for a tool that's growing and getting better over time, Buttondown is a better choice. (Which is not even to speak of all of the other features that Buttondown has that Moonmail doesn't, like analytics and custom domains!